Cargo Tank Maintenance Seminar Hart Endorses Rigorous Inspections, Testing for Pressure Vessel Hoses
Feb 1, 1998 12:00 PM
Roger "Kip" Hart, president of Hart Industries, Middletown, Ohio, endorses rigorous inspection and testing programs for hoses used on pressure vessels and recommends consolidating federal rules to eliminate parallel and superfluous regulations.
Hoses and their ancillary equipment are used in loading and unloading liquefied petroleum gas and anhydrous ammonia from MC331 tank transports.
Hart discussed regulations, testing, and inspection during the 14th annual Cargo Tank Maintenance Seminar, held October 27-29, 1997, in Chicago, Illinois.
Hart's remarks stemmed in part from the Department of Transportation's (DOT) Research and Special Programs Administration's (RSPA) interim final rule now in effect for pressure vessel cargo tanks used to transport products such as propane and other liquefied compressed gases. Included in the interim rule are requirements for testing and mounting hoses.
The interim rule resulted from the misapplication of hose couplings, lack of uniform testing and inspection requirements of hoses, combined with some design malfunctions of excess flow control valves, Hart said.
"In general, most companies involved in the handling of these products know there are dangers and have sound inspection and test programs," he said. "But information, references, and guidelines can help those who don't recognize the need for a sound hose testing and inspection program - the goal being to reduce, and hopefully eliminate, the number of product releases due to failed hoses."
Five Program Components Reviewed Five program components being considered by DOT include identification, inspection, periodic pressure testing, daily inspection prior to use, and records of inspection and testing activities. Although Hart endorses the recommendations, he noted discrepancies between those found in the Rubber Manufacturers Association (Bulletin #IP-11-2) and the newer RMA draft now being written.
A proposal to the RSPA by The Fertilizer Institute calls for hose inspection frequencies one time per year, while RMA recommends one time every day prior to use. Differences are evident in hose-testing frequencies. The Fertilizer Institute recommends one time per year, while RMA calls for tests every 90 days for the first year and once every 30 days thereafter.
In hose-testing pressures, The Fertilizer Institute suggests 80 percent of tank maximum allowable working pressure (MAWP), usually 200-250 poundsper square inch. The RMA recommends twice hose MAWP, 700 psi.
If various federal and industry groups could agree on common standards, then similar guidelines could be put in place, he said. The logic of parallel standards for anhydrous ammonia and LP gas is reasonable because of the similarity of working pressures, tank vessels used for transport, valves, and hose working pressures. Although different types of hoses are required for the two products, whatever guidelines were chosen could apply to both hoses.
"Because of the daily rigors involved in the usage and handling of these types of transport hoses, and the unpredictable variables that they are subjected to, a worst-case approach should always be taken," Hart said.
Variables Cause Problems Variables can cause higher-than-normal pressure surges driven by abrupt valve closure or line blockage, malfunctioning of unloading pump bypass valve, and thermal expansion caused by high ambient temperature.
Surges due to thermal expansion and contraction, as well as pressurization and depressurization, can cause loosening of end fittings. In addition, damage to hose carcasses from kinking, crushing, bending, and flexing may not be apparent during visual inspection.
"It is for these reasons that the RMA's test pressures are set so high. The hose is the weak link and is responsible for many releases. Pressure tests and regular inspections should be conducted on all hoses used to unload and load tanks with hazardous or volatile cargoes," he said.
Recent hose failures have spotlighted the need for mandated testing, Hart said. Government, industry trade groups, and hose manufacturers have focused attention on testing requirements and ways to produce secure end-of-hose fitting attachments. Band clamps, crimped ferrules, and bolted clamps remain the most popular methods of hose coupling attachment.
Interlocking Clamp Advocated "Whatever clamp or ferrule style is used in any compressed-gas or high-pressure application, it is important that it be interlocking by design," Hart said.
The band clamp is used normally in hoses rated at, or below, 150-200 psi MAWP. It is not recommended for LP gas or anhydrous ammonia.
The preferred arrangement is the crimped ferrule design, Hart said, because it provides a permanent hose attachment that virtually eliminates chances of coupling ejection or blowoff. Crimped couplings are designed to hold beyond the rated bursting pressure of the hose, typically five times the MAWP for LP gas and anhydrous ammonia.
Bolted clamps have been the traditional method of choice because of their easy-to-reuse bolted-style hose fitting. "However, if special care is not taken to assure the tightness of these clamps prior to each use, the resulting failure caused by a loose clamp can be catastrophic," he said.
Hart said more information about recommended practices and procedures can be obtained through many hose manufacturers and the Rubber Manufacturers Association.
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